This invention relates to a method and apparatus for the reliquefaction of a compressed vapour, particularly a method and apparatus which are operable on board ship to reliquefy natural gas vapour.
Natural gas is conventionally transported over large distances in liquefied state. For example, ocean going tankers are used to convey liquefied natural gas from a first location in which the natural gas is liquefied to a second location in which it is vaporised and sent to a gas distribution system. Since natural gas liquefies at cryogenic temperatures, i.e. temperatures below xe2x88x92100xc2x0 C., there will be continuous boil-off of the liquefied natural gas in any practical storage system. Accordingly, apparatus needs to be provided in order to reliquefy the boiled-off vapour. In such an apparatus a refrigeration cycle is performed comprising compressing a working fluid in a plurality of compressors, cooling the compressed working fluid by indirect heat exchange, expanding the working fluid, and warming the expanded working fluid in indirect heat exchange with the compressed working fluid, and returning the warmed working fluid to one of the compressors. The natural gas vapour, downstream of a compression stage, is at least partially condensed by indirect heat exchange with the working fluid being warmed. One example of an apparatus for performing such a refrigerant method is disclosed in U.S. Pat. No. 3,857,245.
According to U.S. Pat. No. 3,857,245 the working fluid is derived from the natural gas itself and therefore an open refrigeration cycle is operated. The expansion of the working fluid is performed by a valve. Partially condensed natural gas is obtained.
The partially condensed natural gas is separated into a liquid phase which is returned to storage and a vapour phase which is mixed with natural gas being sent to a burner for combustion. The working fluid is both warmed and cooled in the same heat exchanger so that only one heat exchanger is required. The heat exchanger is located on a first skid-mounted platform and the working fluid compressors on a second skid-mounted platform.
Nowadays, it is preferred to employ a non-combustible gas as the working fluid. Further, in order to reduce the work of compression that needs to supplied externally, it is preferred to employ an expansion turbine rather than a valve in order to expand the working fluid.
An example of an apparatus which embodies both these improvements is given in WO-A-98/43029. Now two heat exchangers are used, one to warm the working fluid in heat exchange with the compressed natural gas vapour to be partially condensed, and the other to cool the compressed working fluid. Further, the working fluid is compressed in two separate compressors, one being coupled to the expansion turbine.
WO-A-98/43029 points out that incomplete condensation of the natural gas vapour reduces the power consumed in the refrigeration cycle (in comparison with complete condensation) and suggests that the residual vapourxe2x80x94which is relatively rich in nitrogenxe2x80x94should be vented to the atmosphere. Indeed, the partial condensation disclosed in WO-A-98/43029 follows well known thermodynamic principles which dictate that the condensate yield is purely a function of the pressure and temperature at which the condensation occurs.
Typically, the liquefied natural gas may be stored at a pressure a little above atmospheric pressure and the boil-off vapour may be partially condensed at a pressure of 4 bar. The resulting partially condensed mixture is typically flashed through an expansion valve into a phase separator to enable the vapour to be vented at atmospheric pressure. Even if the liquid phase entering the expansion valve contains as much as 10 mole per cent of nitrogen at 4 bar, the resulting vapour phase at 1 bar still contains in the order of 50% by volume of methane. In consequence, in a typical operation, some 3000 to 5000 kg of methane may need to be vented daily from the phase separator. Since methane is recognised as a greenhouse gas such a practice would be environmentally unacceptable.
It is therefore desirable to return any flash gas and any uncondensed vapour to the LNG storage tanks of the ship with the condensate. The return of vapour to the storage tanks would in turn tend to enhance the mole fraction of nitrogen in the ullage space of the storage tanks and thereby give rise to two disadvantages. First, as the concentration of nitrogen in the boil-off gas rises, so more work needs to be performed to condense a given proportion of the boil-off gas. Second, variations in the composition of the boil-off gas make the refrigeration cycle more difficult to control.
The method and invention according to the invention are aimed at mitigating the problems that are caused when vapour is returned with condensed natural gas to a liquefied natural gas (LNG) storage tank.
According to the present invention a method of reliquefying vapour boiled off from liquefied natural gas held in a storage tank comprising compressing the vapour, at least partially condensing the compressed vapour, and returning the condensate to the storage tank, wherein the boiled off vapour is mixed upstream of the compression with liquefied natural gas.
The invention also provides apparatus for reliquefying vapour boiled-off from liquefied natural gas held in a storage tank comprising, the apparatus comprising a flow circuit comprising a vapour path extending from the tank through a compressor to a condenser for at least partially condensing compressed boiled-off vapour and a condensate path extending from the condenser back to the storage tank, wherein the apparatus additionally comprises a conduit for the flow of liquefied natural gas into at least one mixer forming part of the flow circuit upstream of (i.e. on the suction side of) the compressor.
Preferably, the flow of liquefied natural gas is taken from storage, or from the condensate itself en route to storage.
There are various advantages given by the method and apparatus according to the invention. In particular since the nitrogen mole fraction in the liquefied natural gas is less than the nitrogen mole fraction in the boiled-off vapour and even less than that in flash gas formed by the expansion through the valve of the condensed boil-off vapour, dilution of the boiled-off vapour with the liquefied natural gas tends to dampen swings in the composition of the vapour phase in the storage tank that would otherwise occur were the characterising feature of the method and apparatus according to the invention to be omitted. Dilution of the vapour upstream of the compressor makes it possible to reduce fluctuations in the work of compression arising from fluctuations in the temperature of the vapour. These fluctuations arise mainly from changes in the loading of the storage tanks. Preferably, the inlet temperature of the boiled-off vapour to the compressor is maintained substantially constant. If desired, there is an absorber of liquid droplets at a position upstream of the inlet to the compressor so as to remove any residual droplets of liquid hydrocarbon arising from the mixing of the vapour with the liquefied natural gas at the second location though generally this measure will not be necessary. Mixing upstream of the compression is particularly important when the storage tank is only lightly laden with LNG, for example after the main part of the LNG has been off-loaded. During normal operation however, it is preferred to perform the mixing with a stream of LNG that is diverted from the condensation path. It then becomes unnecessary to employ any mechanical pump to withdraw LNG from storage for the purposes of temperature control.
There are a number of different preferred additional locations for effecting the mixing of the boiled-off vapour or its condensate with the liquefied natural gas. A first preferred additional location is downstream of the boiled-off vapour compressor but upstream of the inlet to the condenser for the vapour. Preferably, the mixing at this location is controlled so as to maintain a constant vapour temperature at the inlet to the condenser. By so controlling the temperature it is possible to reduce fluctuations in the demand for refrigeration of the condenser which can particularly arise from changes in the volume of liquefied natural gas being held in the storage tank.
Preferably, in order to effect the mixing at this additional location, a second mixing chamber is provided with a first inlet for the vapour and a second inlet for liquefied natural gas in finely divided form. Preferably, the second inlet has a flow control valve associated with it, the position of the second flow control valve being automatically adjustable so as to maintain the temperature of the vapour at the inlet to the condenser substantially constant.
Another preferred additional location for the mixing is downstream of the condenser. More preferably, this other additional location is downstream of an expansion valve or pressure regulating valve in the condensate path. Accordingly the pressure of the condensate is preferably reduced upstream of the other additional location.
If desired, the mixing may be performed at more than one of the above mentioned additional locations. Indeed, it is sometimes preferred that it be performed at both of the above mentioned locations in addition to upstream of the compressor, particularly when the storage is only lightly laden with LNG. During normal, fully laden operation, however, mixing need take place only at a location upstream of the compression.
Preferably, the condensate is returned to the storage tank at a position below the surface of the liquid stored therein. It is desirable to introduce gas bubbles in the returning condensate in to the liquid phase in finely divided form so as to facilitate dissolution of residual uncondensed gas or flash gas formed as a result of the passage of the condensate through the expansion valve.
Preferably, the condenser is cooled by a refrigerant flowing in an essentially closed refrigeration cycle which preferably comprises compressing a working fluid in at least one working fluid compressor, cooling the compressed working fluid by indirect heat exchange in a heat exchanger, expanding the cooled working fluid in at least one expansion turbine, warming the expanded working fluid by indirect heat exchange in the condenser, the working fluid thereby providing refrigeration to the condenser, and returning the warmed expanded working fluid through the heat exchanger to the working fluid compressor.
Preferably the apparatus according to the invention comprises a first support platform on which a first pre-assembly including the condenser is positioned and a second support platform on which a second pre-assembly is positioned, the second pre-assembly including the working fluid compressor, the expansion turbine and the heat exchanger. Alternatively the heat exchanger may form part of a third pre-assembly separate from the working fluid compressor and the expansion turbine. The second pre-assembly can be located in the engine room, or a specially ventilated cargo motor room in the deck house, of an ocean going vessel on which the apparatus is to be used. In these locations, the safety requirements that the compressor and the expansion turbine are required to meet are not as high as in other parts of the ship, for example an unventilated cargo machinery room. Preferably both pre-assemblies are mounted on respective platforms that are typically ship-mounted.
Further, by locating the working fluid compressor and the expansion turbine on the same platform as one another, they can be incorporated in to a single machine. Not only does employing a single working fluid compression/expansion machine simplify the apparatus, it also facilitates testing of the machinery prior to assembly of the apparatus according to the invention on board ship. If desired, a plurality of such compression/expansion machines may be provided in parallel, typically with only one operating at any one time. Such an arrangement enables continuous operation of the working fluid cycle even if it is needed to take a machine in operation off-line for maintenance. The first pre-assembly is preferably located in the cargo machinery room within the deck house of the ocean going vessel. The first pre-assembly preferably includes the or each chamber in which the mixing of the boiled-off natural gas vapour, either upstream or downstream of the condensation, or both, with liquid natural gas from storage is performed. Alternatively the mixing chambers can be installed on board the ship.
Preferably the working fluid compressor and the expansion turbine employ seals of a kind which minimise leakage of working fluid out of the working fluid cycle.
Accordingly, instead of conventional labyrinthine seals, either dry gas seals or floating carbon ring seals are used. Even so, it is desirable that the apparatus includes a source of make-up working fluid. By minimising the loss of working fluid, the amount of make-up working fluid that is required is similarly minimised. Since the working fluid is typically required at a pressure in the range of 10 to 20 bar (1000 to 2000 kPa) on the low pressure side of the cycle, this helps to keep down the size of any make-up working fluid compressor that might be required. If nitrogen is selected as the working fluid, a source of nitrogen which is already at the necessary pressure may be employed so as to obviate the need for any make-up working fluid compressor whatever. For example, the source of the make-up nitrogen may be a bank of compressed nitrogen cylinders or, if the ship is provided with a source of liquid nitrogen, a liquid nitrogen evaporator of a kind that is able to produce gaseous nitrogen as a chosen pressure in the range of 10 to 20 bar. Such liquid nitrogen evaporators are well known. If desired, a third pre-assembly comprising the make-up working fluid supply means on a third platform may be employed.